.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\" Modified 2001-11-28, by Michael Kerrisk, <mtk.manpages@gmail.com>
.\"	Changed data type of proj_id; minor fixes
.\"	aeb: further fixes; added notes.
.\"
.TH FTOK 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
ftok \- convert a pathname and a project identifier to a System V IPC key
.SH SYNOPSIS
.nf
.B #include <sys/ipc.h>
.fi
.PP
.BI "key_t ftok(const char *" pathname ", int " proj_id );
.SH DESCRIPTION
The
.BR ftok ()
function uses the identity of the file named by the given
.I pathname
(which must refer to an existing, accessible file)
and the least significant 8 bits of
.I proj_id
(which must be nonzero) to generate a
.I key_t
type System V IPC key, suitable for use with
.BR msgget (2),
.BR semget (2),
or
.BR shmget (2).
.PP
The resulting value is the same for all pathnames that
name the same file, when the same value of
.I proj_id
is used.
The value returned should be different when the
(simultaneously existing) files or the project IDs differ.
.SH RETURN VALUE
On success, the generated
.I key_t
value is returned.
On failure \-1 is returned, with
.I errno
indicating the error as for the
.BR stat (2)
system call.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR ftok ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
On some ancient systems, the prototype was:
.PP
.in +4n
.EX
.BI "key_t ftok(char *" pathname ", char " proj_id );
.EE
.in
.PP
Today,
.I proj_id
is an
.IR int ,
but still only 8 bits are used.
Typical usage has an ASCII character
.IR proj_id ,
that is why the behavior is said to be undefined when
.I proj_id
is zero.
.PP
Of course, no guarantee can be given that the resulting
.I key_t
is unique.
Typically, a best-effort attempt combines the given
.I proj_id
byte, the lower 16 bits of the inode number, and the
lower 8 bits of the device number into a 32-bit result.
Collisions may easily happen, for example between files on
.I /dev/hda1
and files on
.IR /dev/sda1 .
.SH EXAMPLES
See
.BR semget (2).
.SH SEE ALSO
.BR msgget (2),
.BR semget (2),
.BR shmget (2),
.BR stat (2),
.BR sysvipc (7)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
